Experiments were carried out for the determination of the evaporation and condensation coefficient of water. The system used consists of a thin liquid sheet of water flowing at high velocity in a saturated steam atmosphere. Values of the coefficient α are deduced from the interfacial heat-transfer resistance. This is determined by interferometric measurement of the thickness variation in the sheet due to phase change. Experiments at a steam temperature of about 50°C yield the value α = 0·20 ± 0·04. This result, though indicating relatively high values of α, contradicts the claim that α has the magnitude of unity.